From d8edccdab4a542781477adc1d1dd6cea59d0b94a Mon Sep 17 00:00:00 2001
From: Grant Sanderson <grant@3blue1brown.com>
Date: Fri, 16 Aug 2024 14:59:18 -0500
Subject: [PATCH] Fix zero stroke width issue

---
 manimlib/shaders/quadratic_bezier_stroke/frag.glsl | 11 ++++++-----
 manimlib/shaders/quadratic_bezier_stroke/geom.glsl | 13 +++++++------
 2 files changed, 13 insertions(+), 11 deletions(-)

diff --git a/manimlib/shaders/quadratic_bezier_stroke/frag.glsl b/manimlib/shaders/quadratic_bezier_stroke/frag.glsl
index e1013b3a..7fb668aa 100644
--- a/manimlib/shaders/quadratic_bezier_stroke/frag.glsl
+++ b/manimlib/shaders/quadratic_bezier_stroke/frag.glsl
@@ -1,17 +1,18 @@
 #version 330
 
 // Value between -1 and 1
-in float scaled_signed_dist_to_curve;
-in float anti_alias_prop;
+in float dist_to_curve;
+in float half_stroke_width;
+in float half_anti_alias_width;
 in vec4 color;
 
 out vec4 frag_color;
 
 void main() {
-    if(anti_alias_prop < 0) discard;
+    if(half_stroke_width == 0) discard;
     frag_color = color;
 
     // sdf for the region around the curve we wish to color.
-    float signed_dist_to_region = abs(scaled_signed_dist_to_curve) - 1.0;
-    frag_color.a *= smoothstep(0, -anti_alias_prop, signed_dist_to_region);
+    float signed_dist_to_region = abs(dist_to_curve) - half_stroke_width;
+    frag_color.a *= smoothstep(half_anti_alias_width, -half_anti_alias_width, signed_dist_to_region);
 }
\ No newline at end of file
diff --git a/manimlib/shaders/quadratic_bezier_stroke/geom.glsl b/manimlib/shaders/quadratic_bezier_stroke/geom.glsl
index f8075b8c..ede46580 100644
--- a/manimlib/shaders/quadratic_bezier_stroke/geom.glsl
+++ b/manimlib/shaders/quadratic_bezier_stroke/geom.glsl
@@ -16,8 +16,9 @@ in float v_stroke_width[3];
 in vec4 v_color[3];
 
 out vec4 color;
-out float anti_alias_prop;
-out float scaled_signed_dist_to_curve;
+out float dist_to_curve;
+out float half_stroke_width;
+out float half_anti_alias_width;
 
 // Codes for joint types
 const int NO_JOINT = 0;
@@ -167,8 +168,8 @@ void emit_point_with_width(
 
     // Set styling
     color = finalize_color(joint_color, point, unit_normal);
-    float aaw = anti_alias_width * pixel_size;
-    anti_alias_prop = (width == 0) ? -1.0 : 2 * aaw / (width + 2 * aaw);
+    half_anti_alias_width = 0.5 * anti_alias_width * pixel_size;
+    half_stroke_width = 0.5 * width;
 
     // Figure out the step from the point to the corners of the
     // triangle strip around the polyline
@@ -178,8 +179,8 @@ void emit_point_with_width(
     // The frag shader will receive a value from -1 to 1,
     // reflecting where in the stroke that point is
     for (int sign = -1; sign <= 1; sign += 2){
-        scaled_signed_dist_to_curve = sign;
-        emit_gl_Position(point + 0.5 * (width + aaw) * sign * step);
+        dist_to_curve = sign * (half_stroke_width + half_anti_alias_width);
+        emit_gl_Position(point + dist_to_curve * step);
         EmitVertex();
     }
 }